The present embodiments relate to a local screen for the screening out of magnetic resonance signals of an object under examination during magnetic resonance imaging with a magnetic resonance device.
Imaging systems in medical technology are acquiring a significant role in the non-invasive examination of patients. The representations produced by the imaging systems of the internal organs and structures of the patient are used for the diagnosis of the causes of illness, the planning of operations, the performance of operations, or for the preparation of therapeutic measures. Examples of such imaging systems are ultrasound systems, X-ray computer tomography (CT) systems, positron emission tomography (PET) systems, single-photon emission tomography (SPECT) systems, or magnetic resonance (MR) systems.
With the MR systems, the patient may be subjected to a constant main magnetic field with a relatively high magnetic field strength (e.g., the B0 field). In addition to this, in order to allow for local resolution during the imaging, magnetic gradient fields are overlaid in the different spatial directions. In this situation, the gradient fields may be varied in field strength during the image data acquisition. From time to time, the gradient fields are also activated or deactivated. The variation in the gradient fields, in medical practice, may function at frequencies in the kHz range. In addition to the main magnetic field and the magnetic gradient fields, the patient is also subjected to a pulsed high-frequency field in the MHz range. This field, also designated as the MR excitation field, causes an excitation of the nuclear spin of the atoms or molecules (e.g., the hydrogen protons) in the body of the patient for nuclear spin resonance. After deactivation of the MR excitation field, the excited materials emit MR signals that are recorded by suitable receiver antennae. Due to the fact that the hydrogen protons are found predominantly in the regions of the patient's body that are rich in water and fat, by subsequent calculation operations by an MR system, slice images (tomograms), in which, for example, the regions that are richer in water and fat are represented as lighter than regions that are poorer in water and fat, such as bony structures, may be produced.
For the receiving of the MR signals of the object under examination, local coils are used. The local coils are MR receiving antennae modules that contain MR receiving antenna elements (e.g., conductor loops). The MR signals received by the MR receiving antenna elements are pre-amplified while still in the local coil, are conducted out of the central area of the MR system, for example, via cables, and conducted to a screened receiver of an MR-signal processing device. In this device, the received data is digitalized and further processed for the MR imaging.
During the examination, the local coils are arranged relatively close to the surface of the body, as far as possible directly on the organ or part of the body of the patient that is to be examined. By contrast with larger antennae arranged further away from the patient, which are likewise used for producing an overall sectional image through a patient, the local coils have the advantage that the local coils are arranged closer to the regions of interest. As a result, the noise proportion caused by the electrical losses inside the patient's body is reduced, which leads to the signal-to-noise ratio (SNR) of a local coil being in principle better than that of a more remote antenna. The signal-to-noise ratio may, however, be interfered with by signals emitted from regions of the patient's body that do not belong to the region of interest. This is due to the fact that the high-frequency excitation of the protons of the object under examination during the MR imaging may put into effect by a whole-body antenna, the “remote body array” or “remote body coil”. This provides that the regions of the patient's body that are outside the region of interest are also jointly excited. MR signals that impair the measuring of the MR signals from the regions of interest are accordingly also emitted from these regions. For example, this may lead to backfolding or infolding artifacts in the MR image data if the regions of the body that are not of interest do not lie within the field of view, but nevertheless still produce MR signals in the local coils. Such artifacts in MR imaging occur, for example, in the examination of the abdomen or of the heart, since interfering MR signals are emitted from the arms of the patient, which are spatially relatively close to these regions of the body.